1. Field of the Invention
This invention relates to heat exchangers generally, and, more particularly, to heat exchange processes and to heat exchangers that contain and utilize fluidized small solid particles to improve the transfer of heat on one side of the wall that separates two fluids.
2. Background Art
A common method of exchanging heat between fluids is to position an enclosure of one of the fluids within an enclosure of a second fluid. Then, by directing the fluids through their respective enclosure, heat is transferred from the hotter fluid to the colder fluid. This type of device is commonly referred to as a heat exchanger. Where one of the fluids involved in a commercial heat exchanger is a gas, such as air, the overall transmission coefficient is in the range from 2 to 10 BTU/hr.degree. F. ft.sup.2 (i.e. British thermal unit per hour-degree Fahrenheit). With such a low heat transfer coefficient, commercially available heat exchangers are built with large areas, such as finned or wrinkled tubes, that also require large temperature differences to effectively transfer heat. The users of such heat exchangers are forced to generate large temperature differences, thus making the use of the heat exchanger less energy efficient.
Much higher heat transfer rates have been reported for surfaces immersed in small solid particles, such as sand particles, that are suspended and kept in motion by an upward flow of a fluid. The heat transfer coefficient for these type of heat exchangers can be as high on average as 225 to approximately 250 BTU/hr.degree. F. ft.sup.2. Some heat exchanger systems that immerse surfaces in small solid particles are shown, for example, in U.S. Pat. No. 5,634,516 to Myohanen entitled Method and Apparatus for Treating or Utilizing a Hot Gas Flow, U.S. Pat. No. 5,568,834 to Korenberg entitled High Temperature Heat Exchanger, U.S. Pat. No. 5,533,471 to Hyppanen entitled Fluidized Bed Reactor and Method of Operation Therefor, and U.S. Pat. No. 4,580,618 to Newby entitled Method and Apparatus for Cooling a High Temperature Waste Gas Using a Radiant Heat Transfer Fluidized Bed Technique.
Most heat exchangers that have heat transfer coefficients in the range from 35 to 50 BTU/hr.degree. F. ft.sup.2 use conventional round tubes or pipes. As opposed to the flat surfaces often used to obtain higher rates of heat transfer. Small solid particles make only line or point contact with rounded surfaces. Thus, the amount of heat conducted from or to the small solid particles in contact with rounded surfaces is limited to a small area of contact. It is natural that the studies that used rounded surfaces reported the lower rates and that the studies that used flat surfaces reported that higher rates.
Some heat exchangers allow the fluidized small solid particles to flow into or out of the heat exchanger, as shown, for example, in U.S. Pat. No. 5,347,953 to Adbulally entitled Fluidized Bed Combustion Method Utilizing Fine and Coarse Sorbent Feed, U.S. Pat. No. 5,320,168 to Haight entitled Heat Exchange System for Processing Solid Particulates, U.S. Pat. No. 5,314,008 to Garcia-Mallol entitled Fluid-Cooled Jacket for an Air-Swept Distributor, U.S. Pat. No. 4,862,954 to Hellio entitled Exchanger and Method for Achieving Heat Transfer From Solid Particles, U.S. Pat. No. 4,823,739 to Marcellin entitled Apparatus for Control of the Heat Transfer Produced in a Fluidized Bed, U.S. Pat. No. 4,796,691 to Large entitled Fluidized Bed Heat Exchange Apparatus, U.S. Pat. No. 4,674,560 to Marcellin entitled Process and Apparatus for Control of the Heat Transfer Produced in a Fluidized Bed, U.S. Pat. No. 4,580,618 to Newby entitled Method and Apparatus for Cooling a High Temperature Waste Gas Using a Radiant Heat Transfer Fluidized Bed Technique, U.S. Pat. No. 4,561,385 to Cross entitled Fluidized Bed Shell Boilers and U.S. Pat. No. 4,450,895 to Meunier entitled Process and Apparatus for Heating or Cooling Light Solid Particles.
Some heat exchangers use the downward flow of particles caused by gravity to circulate the small solid particles, as shown, for example, in U.S. Pat. No. 5,601,039 to Hyppanen entitled Method and Apparatus for Providing a Gas Seal in a Return Duct and/or Controlling the Circulating Mass Flow in a Circulating Fluidized Bed Reactor, U.S. Pat. No. 5,000,255 to Pflum entitled Fluidized Bed Heat Exchanger, and U.S. Pat. No. 4,522,252 to Klaren entitled Method of Operating a Liquid--Liquid Heat Exchange.
Many different types of heat exchangers have been developed over the years. U.S. Pat. No. 5,181,558 to Tsuda entitled Heat Exchanger mentions employing a coating film on heat exchanger fins to cause water droplets to more easily roll down the fin rather than bead. Both U.S. Pat. No. 5,109,918 to Huschka entitled Device for the Thermal Treatment of Organic and Inorganic Substances and U.S. Pat. No. 4,423,558 to Meunier entitled Device for Heat Exchange Between Solid Particles and a Gas Current show using burners to heat the small solid particles. U.S. Pat. No. 5,000,255 to Pflum entitled Fluidized Bed Heat Exchanger shows creating a circulating pattern by making the distance between the distributor plate and the tube inlets greater than or equal to five times the diameter of the particles. U.S. Pat. No. 4,971,141 to Kasahara entitled Jet Stream Injection System mentions using slits or slots below round heat exchanger tubes to inject the fluidizing fluid. U.S. Pat. No. 5,143,708 to Nakazawa entitled Tetracosahedral Siliceous Particles and Process for Preparation Thereof shows using a primary particle size of 0.1 to 50 .mu.m. U.S. Pat. No. 4,719,968 to Speros entitled Heat Exchanger mentions a fluidized bed that has small solid particles that are packed together and only allows the fluid through the particle pack via interstitial passageways. U.S. Pat. No. 4,472,358 to Khudenko entitled Packing for Fluidized Bed Reactors shows using various devices to suppress a bubbling particle bed. U.S. Pat. No. 4,561,385 to Cross entitled Fluidized Bed Shell Boilers mentions burning fuel in the particle bed material. U.S. Pat. No. 4,119,139 to Klaren entitled Heat-Exchanger Comprising a System of Granulate Containing Vehicle Tubes, and a Method For Operating the Same shows a heat exchanger that used vertical tubes to catch particles that are fed cyclically into the top and then fall down the tube while increasing in size. U.S. Pat. No. 4,096,214 to Percevaut entitled Multicellular Reactor With Liquid/Gas Phase Contacts mentions a heat exchanger that brings a fluid in contact with a gas during the heat exchange process. U.S. Pat. No. 3,902,550 to Martin entitled Heat Exchange Apparatus shows a heat exchange apparatus that has heating elements or coils in a fluidized bed. U.S. Pat. No. 3,897,546 to Beranek entitled Method of Cooling or Heating Fluidized Beds shows the combustion of fuels using two fluidized particle beds. U.S. Pat. No. 3,814,176 to Seth entitled Fixed-Fluidized Bed Dry Cooling Tower mentions using larger particles embedded within a bed of smaller particles.